Varying a scanning beam as a function of bearing temperature of an optical lens actuator

ABSTRACT

A circuit arrangement for scanning optical recording carriers comprises a scanning device, a first control loop for focusing a light beam of the scanning device directed onto the information plane of the recording carrier and a second control loop for guiding the light beam on an information track. The first control loop exhibits a first control amplifier which is supplied with focus error signals and the second control loop exhibits a second control amplifier which is supplied with track error signals. The control amplifiers drive actuators which mechanically guide the scanning device. To compensate for the temperature response of the control gain, controlling elements connected to temperature sensors via gain controllers are arranged in the first and second control loop, which controlling elements temperature-dependently change the gains.

This application is a national phase filing under 35 USC §371 ofinternational application PCT/DE91/00746, filed Sep. 17, 1991.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a circuit arrangement for scanning opticalrecording carriers.

2. Description of Related Art

In such a circuit arrangement, data which are stored in informationtracks on the information plane of the optical recording carrier areaccessed by a scanning device of a laser light source, optics and agroup of photodetectors. A scanning light beam emitted by the lightsource is reflected by the information plane and directed through theoptics onto the photodetectors. By evaluating the signals of theindividual photodetectors, information signals are obtained and suppliedto subsequent signal processing circuits and devices. The informationsignals can be audio, video or data signals.

To achieve a correct reproduction of the information signals, bothaccurate focusing of the light beam directed onto the information planeof the recording carrier and precise guidance of the light beam alongthe information tracks is required. Apart from a coarse drive which canbe constructed, for example, as spindle in the case of a disc-shapedrecording carrier, there is precision tracking by the scanning device inthat actuators preferably constructed as electromagnetic microdrivesmechanically move its optics. The optics are moved along their opticalaxis for focusing and shifted perpendicularly to the optical axis ortilted by a small angle for tracking.

The scanning device tracking is initiated by focus error signals andtrack error signals which are obtained by further evaluation of thesignals of the individual photodetectors. The scanning device, theactuators and additionally provided control amplifiers form controlloops for the guidance of the scanning device, namely a first controlloop for the focus correction and a second control loop for the trackcorrection.

When the control loops operate optimally, the scanning device is alwaysdisplaced in such a manner that its deviation from the ideal position isjust compensated for. This operating mode of the control loops issignificantly determined by the gains in the overall control loops, thecontrol gains. To compensate for tolerances of control loop elements,gain controllers are provided by means of which the control gain is setduring calibration and adjustment measures during the manufacture of thedevice.

It has been found in practice that in spite of careful setting of thegain controllers, correct reproduction of the information signals is nolonger possible if the circuit arrangement is to be operated under otherenvironmental conditions, particularly other temperatures, thanprevailed during the calibration of the control gain. This state canoccur even if a device with such a circuit arrangement, for example a CDplayer, is to be used in the colder season in unheated rooms, or in theother case is located in a particularly hot environment and in additionis exposed to the heat dissipation of other loads. Especially extremeconditions also occur when a device is operated in motor vehicles.

SUMMARY OF THE INVENTION

The invention is based on the object of eliminating thetemperature-dependent susceptibility to drift of a circuit of the typeinitially mentioned.

The invention is based on the finding that the control gains in thecontrol loops for guiding the scanning device are changed bytemperature-dependent control loop elements. In particular, themechanical bearings in which the optics of the scanning device aremovably supported have been identified as temperature-dependent controlloop elements. The resistance to be overcome by the actuators whenmoving the optics increases with decreasing temperature and vice versa.This leads to it not being possible to achieve adequate guidance of thescanning device at low temperatures whereas control oscillations occurat high temperatures. The consequence of both phenomena is that it isnot possible to guide the scanning device into its ideal position.

Eliminating the temperature-dependence of the bearings by mechanicalmeans would be expensive. However, since the bearings represent controlloop elements in the control system sense, there is the possibility ofelectronically compensating for their effect on the control gain. Forthis purpose, controlling elements are inserted into the control loopswhich are operated in the opposite direction to the change in mechanicalresistance of the bearings. The controlling elements are operated bygain controllers which are supplied with signals from temperaturesensors which detect the environmental temperature of the bearings. Thetemperature-dependence of the bearings can be compensated for when theirtemperature/resistance characteristic is simulated by the gaincontrollers in the oppositely directed sense.

The electronic solution has the additional advantage that othertemperature-dependent influences on the control gain can also becompensated for regardless of their precise origin. There is furthermorethe possibility of exercising control over temperature/resistancecharacteristics which have a non-linear or even discontinuous variation.Temperature/resistance characteristics with discontinuous variationhaving its origin in a temperature-dependent change in the molecularchains occur especially in the case of bearing hinges of plastic parts.Furthermore, it is no longer necessary to maintain a particularoperating temperature during a calibration of the control gain. A devicecalibrated in the cold state also operates correctly within the entiretemperature range for which the compensation is designed.

According to a further development, the controlling elements of thefirst and second control loop are connected to a common gain controller.

If the optics of the scanning device are moved in one and the samebearing both in a displacement for the purpose of focusing and in amovement for tracking or if the scanning device is guided at least insimilar bearings in both types of movement, which will be the case as arule, the same temperature/resistance characteristics can be assumed.The restriction to a common gain controller for both control loops notonly reduces the circuit complexity but also achieves synchronism ininfluencing the control gains in both control loops in a simple manner.This aspect is essential particularly in the case of very complicatedtemperature/resistance characteristics, the simulation of which iscomplex. Where there are different control gains in the control loopsbut qualitatively identical temperature/resistance characteristics, thecontrolling element in the control loop having the characteristic withthe lesser slope can also be driven via a voltage divider from the gaincontroller.

This solution has the advantage that, as the power of the light beamchanges, the control gain is changed jointly in both control loops. Itis thus not necessary here to produce any circuit linking between thecontrol loops and, instead, there is a mandatory optical linking throughthe light beam, from the deviation of which its focal point from theinformation plane or its direction from the information track focuserror signals and track error signals are obtained by evaluating thesignals of the individual photodetectors. The power of the light beaminfluences the magnitude of these error signals.

BRIEF FIGURE DESCRIPTION

Further developments and advantageous embodiments are obtained from theclaims, the further description and the drawing, which representillustrative embodiments of both independent solutions and in whichdrawing:

FIG. 1 shows a block diagram of a first circuit arrangement for scanningoptical recording carriers, and

FIG. 2 shows a block diagram of a second circuit arrangement forscanning optical recording carriers.

DETAILED DESCRIPTION

Common features of the circuit arrangements shown in FIGS. 1 and 2comprise a scanning device 10 consisting of a laser light source 36,optics 38 and a group of photodetectors 40, an evaluating andpreamplifier circuit 42, a control amplifier 16 of a first control loop12 with a first actuator 20 for the focus correction and a controlamplifier 18 of a second control loop 14 with a second actuator 22 forthe track correction.

The circuit arrangements described thus far provide for aninterference-free access to stored information items of an opticalrecording carrier, with the constraint of constant temperatureconditions. A light beam radiated by the laser light source 36 isfocused on the information plane of the recording carrier and directedonto an information track. The light beam reflected from the informationtrack passes to a group of photodetectors 40. The focusing and precisealignment of the light beam on the recording carrier and the imaging onthe photodetectors 40 are carried out by means of the optics 38. Forreasons of simplification, the drawing only shows the virtual beam pathfrom the laser light source 36 via the optics 38 to the photodetectors40. The optics 38 can be mechanically displaced by means of theactuators 20 and 22 in a direction along their optical axis for focusingand in a direction perpendicular thereto for tracking.

If the optics 38 are located in their ideal position, informationsignals can be obtained by means of the evaluating and preamplifiercircuit 42 and processed further by subsequent signal processingcircuits, not shown here. In the case of deviations from the idealposition, error signals are generated by the evaluating and preamplifiercircuit 42. A deviation of the focal point from the information plane bythe amount FSF leads to an error signal FF, the magnitude of whichdepends on the degree of deviation, the power of the laser light sourcePL and the attenuation D of the scanning beam. If the scanning beam doesnot accurately impinge on the information track, an error signal FT isproduced, the magnitude of which is dependent on the deviation FST fromthe track, the scanning power PD and the attenuation D of the scanningbeam.

The error signals FF and FT pass to the control amplifiers 16 and 18,respectively. These generate control signals YF and YT for the actuators20 and 22, respectively, which produce a mechanical displacement of theoptics 38 by the amount SF and ST. In the ideal case, this displacementis exactly large enough for the deviation from the ideal positioncausing it to be just compensated for. For this purpose, the total gainin the respective control loop 12 and 14 respectively, the control gain,as one of the most important parameters of the stability of the control,must vary as little as possible over the entire operating range of thecontrol.

To achieve this characteristic in spite of the temperature-dependentmechanical resistances of the bearings of the optics 38, the circuitarrangement according to FIG. 1 comprises controlling elements 28 and 30in the first 12 and second control loop 14. These controlling elements28, 30, which can also be integrated in the evaluating and preamplifiercircuit 42, are connected to the output of a common gain controller 26which, in turn, is supplied with signals from a temperature sensor 24.The temperature sensor 24 is preferably located in the vicinity of thebearings of the optics 38 or is thermally coupled to these.

The gain controller 26 has a control characteristic which varies in theopposite direction to the temperature/resistance characteristic of thebearings of the optics 38. If it is assumed that the bearings used havethe same characteristics both during the displacement for focusing andduring the movement for tracking, the assumption of identicaltemperature/resistance characteristics in both control loops 12, 14 isapplicable. Using the common gain controller 26 therefore makes itpossible to keep down the additional circuit expenditure and theexpenditure for simulating the temperature/resistance characteristics.

The temperature sensor 24 transmits for each temperature value within atemperature range for which the compensation has been dimensioned asignal value to the gain controller 26. This controller allocates, inaccordance with its control characteristic, a control value to thetemperature value determined, which control value is used for settingthe controlling elements 28, 30 in the control loops 12, 14. Thecontrolling elements 28, 30 thus compensate for the control gain changedby the change in mechanical resistances of the bearings and thus providecontrol stability.

A voltage divider 32 additionally drawn-in in the block diagram makes itpossible to adapt the control of the controlling elements 28, 30 whenthe temperature response in the two control loops 12, 14 has a differentslope.

FIG. 2 shows another solution which achieves a constant control gainover the entire operating range of the control in spite of thetemperature-dependent mechanical resistances of the bearings of theoptics 38. The circuit arrangement shown comprises a controlling element34 which controls the power PL of the laser light source 36. It wouldalso be possible to control the attenuation D of the scanning beam or tocombine both measures.

The controlling element 34 is connected to the output of a gaincontroller 26, analogously to the circuit arrangement in FIG. 1, whichcontroller, in turn, is supplied with signals from a temperature sensor24. This embodiment mandatorily results in a common, qualitativelyidentical control of the control gains in both control loops 12, 14.

Apart from an analogous simulation of the temperature/resistancecharacteristic of the bearings of the optics 38 by means of analogcircuits, it is also possible to construct the gain controller 26 as alook-up table in which a stored output variable is allocated to eachstepped input variable. This makes it possible to implement evencomplicated characteristics by simple means.

We claim:
 1. A circuit for recording/reading an optical recordingmedium, comprisinga scanning device (10), said scanning device having afirst control circuit (12) for focusing a light beam directed on aninformation level of the recording medium and a second control circuit(14) for directing the light beam along an information track at saidinformation level, wherein a temperature sensor (24) detects thetemperature of bearings of an optical element of the scanning device(10), the mechanical mobility of which bearings istemperature-dependent, and wherein servo amplifiers (16, 18) of thecontrol circuits (12, 14) trigger a first actuator (20) for focusing anda second actuator (22) for tracking,wherein the first and secondactuators (20, 22) mechanically guide the optical element (38) of thescanning device (10), and a beam power controlling device (34) isprovided, by means of which the intensity (PL) of the light beamdirected on the recording medium is varied as a function of thetemperature detected by the temperature sensor (24).
 2. A circuit inaccordance with claim 1, whereinsaid beam power controlling device (34)is controlled by a gain control (26) having a characteristic curve whichcompensates for temperature drift of the servo amplifiers (12, 14) overa predetermined temperature range.
 3. A circuit in accordance with claim2, whereinthe gain control (26) is provided in the form of a look-uptable wherein a stored output value is assigned to each stepped value oftemperature of said sensor.
 4. A circuit in accordance with claim 1,whereinthe gain control (26) is provided in the form of a look-up tablewherein a stored output value is assigned to each stepped value oftemperature of said sensor.